Generally, a CRT indicator having 320 dots in a vertical direction and 240 dots in a horizontal direction is used for this type of apparatus as shown in FIG. 7. As one display mode, character information such as distance, bearing, azimuth, range and the like are displayed in an upper portion of the indicator having 24 dots in a vertical direction and in a lower portion thereof having 40 dots in the vertical direction, and a portion having 240.times.240 dots is used as a net display area in the remaining area of 256 (V).times.240 (H) on the indicator to display searched information representative of underwater conditions horizontally extending from the own ship. As modes to present display examples on a display area, there are a normal display mode in which data obtained by searching the water in the whole range of directions are displayed in a circled area with its radius of 120 dots and with the position of own ship "A" (represented with .DELTA. mark) at tile center of the circle as shown in FIG. 7, a dual display mode in which underwater conditions surrounding the own ship are displayed in an upper half portion of the indicator in accordance with bearings of targets with respect to the own ship and distances between the targets and the own ship and echo signals resulting from respective search signals radiated successively in different directions are successively displayed in a lower portion of the indicator in a vertical direction as shown in FIG. 8, and an enlarged display mode in which the position "A" of own ship is moved from the center of the screen (off-centered), and the azimuthal search range is limited to search underwater conditions in a sector form and a searched area is displayed on the screen in a sector form with its radius of 180 dots as shown in FIG. 9. With the enlarged display mode, a range can be increased so that a search area will be large, and objects can be displayed in high density, i.e., in high resolution, by keeping a search area set as the same as with the normal display mode.
Referring to FIG. 9, a black figure "Q" seen in the sectorial search area represents a school of fish. If the fish school "Q" is moving in a right direction on the screen, it will leave the search area soon. Thus, the search area has to be changed in order to track it.
However, a search area (indicated by broken lines) will go out of the screen, if only the center "B" of the turning movement of the transducer (center of turning) indicated with a mark .circle. is changed. In order to fully display a search area with no any parts thereof missing after the search area is changed. the center "B" of turning movement has been changed after the position of own ship "A" had been moved, or the position of own ship "A" has been moved after the center "B" had been changed. Thus, prior art apparatuses have a problem that schools of fish may be lost due to the lack of promptness because at least two setting operations have been required.
FIG. 16A shows an example to search a sectorial area determined by limiting the azimuthal range of the turning movement, with a PPI sonar searching the water in the whole range of directions with respect to the own ship by turning the transmitting and receiving transducer. The position "A" of own ship is represented with a mark .DELTA.: an area "Y.sub.1 " indicated by solid lines is the area being probed; and the center "B.sub.1 " of the turning movement in an area "Y.sub.1 " is represented with a mark .circle. while the search area "Y.sub.1 " being searched. A line "L" in a radial direction is a sweep line and is being turned in a rightward direction. A black figure indicates a school of fish "Q". Thus, the fish school "Q" in the figure has been drawn by the previous scan.
FIG. 16B shows a display in which the sweep line "L" has moved to the right edge. Only a part of the fish school "Q" is being displayed, since the school of fish "Q" had moved in the right direction. Thus, it is a general practice to move the sweep line "L" to the right edge and to issue a command to move the search area in a rightward or a leftward directions after seeing the result of the search in order to track the fish school "Q". It is assumed now that a search area "Y.sub.2 " is newly set. After the search area is changed, the center "B.sub.2 " in turning the transducer is designated with a mark .largecircle.. However. the sweep line "L" has already started to move in a leftward direction when a new search area "Y.sub.2 " is set. The sweep line "L" turns to left even in the probing area "Y.sub.2 " as shown in FIG. 16(C). As a result. if the fish school "Q" continues to move in the right direction, only part of the fish school "Q" is displayed or no fish school is displayed the screen after the search area "Y.sub.2 " is set. Thus, prior art devices have such a disadvantage that in order to display the fish school "Q" next time, one has to wait until the sweep line L turns back from the left edge and the fish school "Q" might be lost during that time period.